In the belt of the radial tire for passenger car are used at least two or more angled steel belt layers at the present time. Also, in order to ensure the stability during the running and particularly prevent the separation of the belt layer during the running at a high speed (particularly separation remarkably caused at the end portion of the belt layer) and improve the durability, it is general to adopt a cap-layer structure.
The cap-layer means a ply formed by continuously and spirally winding in a circumferential direction of the tire, which may be called as a belt reinforcing layer. The “cap-layer” is called as “belt reinforcing layer” hereinafter.
As a cord material of the belt reinforcing layer is generally used nylon. However, nylon is unsuitable from a viewpoint of characteristics recently required in parts of a new product (Original Equipment: OE). As a problem is particularly mentioned road noise, decrease of flat spots, or poor protruding property of the end portion in the belt reinforcing layer during the running at the high speed, and it is demanded to solve such a problem.
As a countermeasure, it is proposed to apply polyketone fibers to the belt reinforcing layer. The polyketone fiber is high in the modulus of elasticity as compared with nylon and has no glass transition temperature (Tg), so that it can expect the effect of controlling the road noise, the decrease of flat spots or the protruding (see, for example, Patent Article 1: JP-A-2000-142025 and Patent Article 2: JP-A-2001-334811).
In general, fibers of polyethylene terephthalate (hereinafter referred to as PET simply) are used in a carcass ply of a radial tire for a passenger car. Also, rayon fibers are used in the carcass ply of tires for ultra-high performance (Ultra High Performance) vehicles recently demonstrating an upward trend or tires having such a size that heat buildup in a side portion of the tire is severe.
However, the PET fibers have a problem in the adhesion property at high temperatures (resistance to thermal adhesion) and may not be followed to the upward tread of the recent high performance vehicles, while the rayon fibers have a difficulty in view of feeding with respect to the upward trend of the high performance vehicles, so that aliphatic polyketone fibers having a high elasticity and an excellent resistance to thermal adhesion are noticed instead of the above two materials (see, for example, Patent Article 3: JP-A-2000-190705, Patent Article 4: JP-A-2000-264012, Patent Article 5: JP-A-2001-334807 and Patent Article 6: JP-A-2002-307908).
The inventor has found that there is a fear of causing the following problem in the conventional techniques (see the Patent Articles 1 and 2).
When the coating rubber in the use of nylon as a belt reinforcing layer is combined with the polyketone fibers as it is, there is produced the effect of controlling the protruding by using the polyketone fibers, but the stepwise difference of the rigidity at the interface between rubber and cord becomes large to bring about the stress concentration and hence the high speed durability (HSP drum level) does not reach to a significant HSP drum-up.
Further, the inventor has found that there is a fear of causing the following problem in the conventional techniques (see the Patent Articles 3-6).
When the aliphatic polyketone fibers are used in the carcass ply, since they are high in the elasticity and excellent in the resistance to thermal adhesion as compared with PET and rayon, it should be expected to largely improve the performances in the evaluation on durability (drum test) applying a load to a region ranging from the side portion to the bead portion in the tire or the like. According to the inventor's studies, however, it has been found that when the coating rubber for a treat in case of using PET and rayon in the carcass ply is combined with the aliphatic polyketone fibers, the stepwise difference of the rigidity at the interface between rubber and cord becomes large to bring about the stress concentration and hence it is not connected to the significant drum-up.
It is an object of the invention to rationalize the rigidity balance between the polyketone fibers and the coating rubber covering them, loss characteristic and the like for solving the above problems.